Cassegrain radar antenna with selectable acquisition and track modes

ABSTRACT

A Schwarzschild Antenna cooperates with an organ pipe scanner to achieve wide angle sectoral scanning of a high gain pencil beam. A rotatable mirror switches the antenna to conical scanning whereby microwave energy communicates between a nutating horn and reflectors of the Schwarzschild Antenna. During conical scanning, the organ pipe scanner remains unenergized. The mode of operation is selectable by the operator and the system is designed for rapid switching.

United States Patent 1191 Meek et al.

1 1 Feb. 26, 1974 CASSEGRAIN RADAR ANTENNA WITI-I SELECTABLE ACQUISITIONAND TRACK MODES [75] Inventors: James M. Meek, Silver Spring;

Clarence F. Ravilious, Rockville; Whilden G. Heinard, Bethesda, all ofMd.

[73] Assignee: The United States of America as represented by theSecretary of the Army, Washington, DC.

[22] Filed: Feb. 26, 1973 [21] App]. No.: 335,877

52 US. Cl 343/761, 343/777, 343/779, 343/781, 343/835, 343/837, 343/83951 1111. C1. HOlg 3/18, HOlg 3/20, HOlg 3/26 [58] Field of Search...343/777, 779, 781, 761, 757, 343/758, 835, 837, 839

[56] References Cited UNITED STATES PATENTS 3,696,432 lO/l972 Andersonet al. 343/76l Primary Examiner-Archie R. Borchelt AssistantExaminer-Marvin Nussbaum Attorney, Agent, or FirmEdward J. Kelly;Herbert Berl; Saul Elbaum [57] ABSTRACT A Schwarzschild Antennacooperates with an organ pipe scanner to achieve wide angle sectoralscanning of a high gain pencil beam. A rotatable mirror switches theantenna to conical scanning whereby microwave energy communicatesbetween a nutating horn and reflectors of the Schwarzschild Antenna.During conical scanning, the organ pipe scanner remains unenergized. Themode of operation is selectable by the operator and the system isdesigned for rapid switching.

10 Claims, 3 Drawing Figures CASSEGRAIN RADAR ANTENNA WITH SELECTABLEACQUISITION AND TRACK MODES The invention described herein may bemanufactured, used, and licensed by or for the United States Governmentfor governmental purposes without the payment to us of any royaltythereon.

FIELD OF THE INVENTION The present invention relates to a CassegrainAntenna, and more particularly to such an antenna with selectableacquisition and track modes.

BRIEF DESCRIPTION OF THE PRIOR ART The prior art relating to microwaveantennas includes a structure known as a Cassegrain Antenna which iscomprised of coaxial reflectors. The Cassegrain has met with wideacceptance because its structure eliminates the need for mounting aheavy feed radiator far in front of the main reflector of the antenna.An improvement of the Cassegrain came with the discovery of an antennastructure known as the Schwarzschild Antenna which is basically amodified Cassegrain with reflectors shaped to form an aplanatic system.As those of skill in the art know, the aplanatic Schwarzschild meets theAbbe sine condition and evidences superior off-axis microwave focusingcapability, when compared with the older, conventional Cassegrain.Although the Schwarzschild Antenna has been designed to operate in theconical scanning mode, there has not been a satisfactory designheretofore capable of effecting rapid switching between this mode and asectoral scan mode in one antenna assembly.

Therefore, in conventional radar systems where relatively wide anglesectoral scanning is required along with conical, or steady tracking, arelatively complicated antenna structure becomes necessitated. A resultof this complexity is that there is a decrease in performancecharacteristics and flexibility.

BRIEF DESCRIPTION OF THE PRESENT INVENTION The present invention isdirected to a Schwarzschild Antenna which cooperates with an organ pipescanner for relatively wide angle unidirectional sectoral scanning. Astructurally simple rotatable mirror acts as a microwave energy switchto deactivate the organ pipe scanner, and instead, complete an energypath between nutating horns and the reflectors of the antenna. Theswitch to the latter described mode effects conical scanning. Thecombination of the Schwarzschild Antenna with an organ pipe scanner isnovel. The further addition of a switching capability to switch theantenna from the sector scan mode to a conical scan mode lends furtheruniqueness to the present invention.

The resultant structure of the present invention provides an improvementin tracking radar antenna systems related to scanning capabilities,multiple operating modes, and beam pattern optimization.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is a side elevational view ofthe present Schwarzschild Antenna illustrating a pivotally mountedmirror which switches in either an organ pipe scanner or a conical scanhorn.

FIG. 2 is a front sectional view taken along a plane passing throughsection line 2 2 of FIG. 1.

FIG. 3 is a rear sectional view taken along a plane passing throughsection line 3 3 of FIG. 1.

This view illustrates the common termination of all pipes in the organpipe scanner.

DETAILED DESCRIPTION OF THE INVENTION Referring to the figures and moreparticularly FIG. 1 thereof, a side elevational view of the presentSchwarzschild Antenna and associated scanners are illustrated.

A sub-reflector 10 is shown in radially spaced parallel relation to amain reflector 12. The reflectors constitute a Schwarzschild system.Although the projections of the reflectors 10 and 12 are circular, thecontours of the reflectors satisfy the design criterion for aplanaticreflector systems meeting the Abbe sine condition. Parallel rays 14 and16, for example, are seen to communicate between the lower portion ofthe main reflector 12 and the far field, while parallel rays 18 and 20,for example, are seen to communicate between the upper portion of themain reflector l2 and the far field. An axis 21 intersects the center ofboth the sub-reflector 10 and the main reflector 12. A rectangleaperture 26 is formed around the intersection of the axis 21 and themain reflector l2.

When receiving microwave energy in the sectoral scan mode, ray 16impinges upon the main reflector 12 at point 22. Ray 16 is illustratedto represent the lower limit of impinging microwave energy. Thereafter,the ray is reflected to the sub-reflector 10 where incidence occurs atpoint 24. Subsequent reflection from point 24 occurs so that the raypasses through the central aperture 26 for formation of a virtual imageat focal point 28.

An image at point 28 never develops because of the intercedingdisposition of a pivotally mounted flat mirror 30. While operating in arelatively wide angle sectoral scan mode, the mirror, which may berectangularly shaped, assumes the solid position indicated by A. Themirror 30 is positioned to reflect the microwave energy after it passesthrough the aperture 26. Therefore, ray 16 is reflected from the lowerpart of the mirror 30 until it is focused at focal point 34. Apyramidshaped horn (flare) 36 surrounds the focal point 34 so that thehorn 36 can feed microwave energy to (or from) connected waveguides.Upper ray 18 travels a similar route. After reflection from the mainreflector at point 37, the ray impinges upon the sub-reflector 10 whereit is further reflected at point 39 through the aperture 26. Afterpassing through the aperture 26, the ray l8 impinges upon the upperportion of mirror 30 and is reflected therefrom to the focal point 34.Thus, the energy associated with the upper beam 18 is likewise fed tothe horn 36.

As clearly shown in FIG. 2, a plurality of adjacentlyspaced pyramidhorns 36 and their connected pipes 40 form an assembly of pipes 38 thatis conventionally referred to as an organ pipe scanner. The hornportions 36 of the organ pipe scanner 38 have outward ends referred toas output flares. These flares lie along a circular arc 41. The edges ofthe horns 36 are positioned as shown, perpendicular to the arc 41.

Each horn 36 has a rectangular cross section and pyramida] or taperedinterior shape.

The inward end of each horn communicates with its own pipe such as 40.The lower end portion of all pipes such as 40 may be twisted as shown bythe rearwardly extending section 42 so as to provide the desireddirection of linear polarization in the far field. The latter mentionedsection 42 articulates to a further perpendicularly disposed pipesection 44 that has its outer end 46 (FIG. 3) communicating with acircular chamber generally indicated by reference numeral 48 in FIG. 3.As indicated by FIG. 3, the openings 46 of the various pipesperpendicularly intersect the circumference of the circular chamber 48and are arranged so that the microwave E planes coincide (coplanar).Otherwise stated, the pipes are radially positioned with respect to thecenter of the circular chamber 48.

A single horn 50 flared in the microwave E plane is mounted about anaxis 54 that is perpendicular to the plane of the circular chamber 48.The axis intersects the center of the circular chamber 48. The outer,outwardly flared end of the horn 50 communicates with several pipeopenings 46 (stacked in the narrow E plane dimension as stated) at agiven instant of time as the horn 50 rotates either clockwise orcounterclockwise. This is due to the relatively large opening 52 of thehorn 50 as compared with a much smaller opening in the end 46 of theradially positioned pipes. As illustrated in FIG. 3,the axis about whichthe feed horn 50 rotates is indicated by reference numeral 54. Theinward end of horn 50 communicates with a waveguide fitting 56 that isconnected to. waveguides (not shown) which deliver microwave energy fromthe antenna to a remote transmitter-receiver.

As the horn 50 rotates in a circular manner, energy is sequentiallycollected from the ends 46 of the pipes. At some point along thecircumference of the circular chamber 46, one of the pipes willrepresent the left most horn 36 in FIG. 2, while an adjacent pipe alongthe circumference 48 in FIG. 3 represents the right most horn in FIG. 2.Accordingly, as the horn 50 rotates circularly, an arcuate scan isproduced in a unitary directional manner at the horns 36. Thisunidirectional scan is indicated by the direction arrow 58 in FIG. 2.The result of this unidirectional scan is a wide angle unidirectionalsector scan by the radar.

Proper illumination of the sub-reflector is accomplished to a largeextent by providing a primary beam width corresponding to the circularextent of the subreflector. Generally, a narrower flare (36) transverseto the direction of scan provides a broader beam and vice versa. In thedirection of scan, the total width of the several horns illuminated at agiven instant is essentially the governing factor. The phasing of outputwaves at these horns should preferably be controlled by adjustingwaveguide electrical path lengths between feed and output, so that theoutput wave is directed toward the center of the subreflector at allscan angles.

It should be understood that the antenna is a. reciprocal device and theray paths illustrated apply for both transmission and reception.

As previously mentioned, the central object of the present invention isto be able to selectably switch from the organ pipe sector scan to asteady monopulse or conical scan. This is done by rotating shaft 64 thatmounts mirror 30. This rotation is achieved by a mirror drive torquer(motor) 65 shown in FIG. 2. The torquer 65 achieves rapid 90 rotation ofthe mirror 30 to position B shown in FIG. 1. As the mirror is shifted tothis new position, the microwave channel to the pipe organ scanner isdisconnected and becomes inoperative. For precise mirror positioning, azero-backlash, motor driven cam mechanism may be employed in lieu of thetorquer mentioned above.

Referring once again to FIG. 1, reference numeral 60 denotes a nutatingpyramid horn that is positioned above the mirror reflector which is nowassumed to be in the dotted position B. Reference numeral 20 is seen todenote the geometric upper limit of received mircowave energy whichimpinges upon the main reflector at point 62. From there, the beam 20 isreflected at point 39 on the sub-reflector 10. Thereafter, the beampasses through the opening 26 where it impinges upon the front surfaceof mirror 30. Reflection from the mirror takes place and the beam 20intersects the focal point 65. In a similar manner, the geometricopposite ray 14 reflects off the main reflector 12 at point 67 forsubsequent impingement upon the sub-reflector 10 at point 24.Thereafter, the ray 14 is directed through the aperture 26 until itimpinges upon a lower portion of the mirror 30. The ray 14 is reflectedfrom this lower portion so that it intersects the focal point 65. Thenutating horn 60 centered at the focus collects these rays, as well asthe rays that are present in between the edge rays 20 and 14. By virtueof the nutating motion, conical scanning is achieved.

During transmit operation of the antenna, the microwave signal flow isoppositely directed as compared to during receive operation.

Thus far, sectoral scan and conical scan have been described. The beamis a high gain pencil or fan beam. The present invention is equallyapplicable to steady tracking or monopulse radar operation. To achievethis monopulse operation, four or more ports are employed. For example,the ports are characterized by pyramid horns including theaforementioned horn 60, horns 66 (FIG. 1), 68 and 70 (FIG. 2). Thesefour fixed horns form approximately a square pattern and transmit andreceive microwave energy in the conventional manner.

Thus described, the present Schwarzschild Antenna is seen to operatewith selectable acquisition and track modes. These modes includeunidirectional organ pipe; wide-angle-sector scan; monopulse steadytrack; and conical scan. The beam may be generally described as eitherpencil beam or oval (fan) beam.

Although the present invention has been discussed in a manner indicatingonly two extreme positions of mirror 30 (position A and B), it should beappreciated that the mirror 30 can be varied in position about theseextreme mirror locations to achieve within certain angular limitationsbi-level or raster scanning patterns of the far-field pencil beam. Thisis achieved, for example, by adjusting the mirror 30 so that it steps toa new scanning plane at the termination of a unidirectional sector scan.This is easily accomplished with state of the art techniques. Instead ofa two-position torquer, a stepping motor or Geneva movement can beemployed, for example.

An additional design consideration for the present invention is directedto the utilization of a twistflector for the main reflector, and atransflector for the subreflector. As those of ordinary skill in the artknow, the twistflector has a grating for changing linear polarizationduring reflection. The transflector allows energy to be propagated orreflected depending on the polarization of incident energy. Inconsideringthe microwave energy path between the reflectors, thetwistflector changes the polarization of the energy impinging thereuponand effects reflection to the sub-reflector on receive. Thesub-reflector then directs the microwave energy through the aperture 26.As is well known in the art, the use of a twistflector-transflectorcombination has the advantages of minimizing side lobes, maximizinggain, ang generally producing an improved pattern due to decreasedblockage.

When designing the antenna of the present invention, certain parametersare specified. Thus, for a particular application one must choose orspecify for example frequency of operation, antenna size, scan angle,gain, beam width, and maximum side lobe level.

In order to convert these given parameters to the antenna componentmeasurements, ray tracings are performed. This is a conventionaltechnique which gives shapes of Schwarzschild reflectors. After thetracings have been made, a computer program is employed to determinebeam pattern parameters for design optimization. That is, the bestcompromise must be made between the desired radiation characteristicsand physical size limitation of antenna geometry. The ray tracingbeampattern parameters may include:

Specification of a Schwarzschild or a regular Cassegrain antenna design.

Antenna diameter to operating wavelength ratio.

Antenna/feed aperture illumination functions.

. Sector scan angle.

Focal length.

Magnification factor.

Diameter of sub-reflector for opaque reflectors (blockage).

Size of aperture in main reflector when using twisttransflector(blockage).

The desired scanner is designed by laying out, as scale drawings,various configurations of waveguide convolutions, fitting scanner feedarc to reflector focal are (from ray tracings). Also feed apertures areinserted in the design drawings and test models to give proper feed beamwidth and directivity to illuminate the sub-reflector.

The following will provide references, in the literature, to theabove-discussed design considerations.

BEAM PATTERN PROGRAM One may use a computer program developed by theRome Air Development Center (Griffiss Air Force Base New York) and TRGCompany (Nihen and Kay). Reference RADC TR-66-582, November 1966,authored by Hildebrand. The reference is entitled Design and Evaluationof Two-Reflector Antenna Systems. This program is modified to permit theuse of quasi-parabolic primary illumination functions and for patternsin scan direction and orthogonal thereto.

RAY TRACING PROGRAM Ray tracing equations may be derived from the basicSchwarzschild reflector equations given in an Airborne InstrumentLaboratory report found in the records of the IRE Convention of 3-20-62,Topics in Electronics, Volume IV, 1963, authors W. White and L. DeSize.The title is Scanning Characteristics of Two-Reflector Antenna Systems."Also reference Radio Engineering and Electronics Physics, USSR, VolumeVI, 1961, authored by N. G. Ponomarev. The title is Graphical Methods ofDesigning Aplanatic Antennas.

OPTIMIZATION OF TWIST-TRANSFLECTOR (GRATING) SYSTEM Wheeler Labs (GreatNeck, New York), Hazeltine Corporation, Greenlawn, New York. The reportis denoted as 666 and is entitled Design of Twistreflector HavingWideband and Wideangle Performance. The report is dated 4-7-55 andauthored by Peter W. Hannan.

ANTENNA APERTURE BLOCKAGE IRE Transactions on Antennas and Propagation,3-12-60. The report is entitled Microwave Antennas Derived from theCassegrain Telescope. The article was authored by Peter. W. l-lannan.

ORGAN PIPE SCANNERS Naval Research Lab Report 3842, dated Aug. 1, 1951.The authors were K. Kelleher and H. Hibbs. The report is entitled AnOrgan Pipe Scanner.

RING FEED SCANNERS Georgia Tech Research Institute Report 212-168; ABNo. 17816; 1953. The report is entitled Two Beam Scanning Antenna."

We wish it to be understood that we do not desire to be limited to theexact details of construction shown and described for obviousmodifications can be made by a person skilled in the art.

Wherefore we claim the following:

1. A Schwarzschild antenna system comprising:

reflector means for reflecting microwave energy that impinges thereon;scanner means adjacent the reflector means and communicating with thereflector means for producing a wide angle unidirectional sectoral scan;

horn means disposed in adjacent spaced relationship to the reflectormeans and communicating with the reflector means for producing a narrowangle scan; and microwave switching means mounted adjacent to thereflector means to selectively complete microwave communication betweenthe reflector means and either the scanner means or the horn means;

whereby the switching means enables rapid selection of the sectoral scanor the narrow angle scan.

2. The structure of claim 1 wherein the scanner means comprises aplurality of adjacently spaced feed horns communicating with respectivewaveguide pipes to form an organ pipe scanner which produces aunidirectional scan across the outlet ends of the feedhorns.

3. The structure of claim 1 wherein the horn means comprises at least asingle nutating horn for conical scan tracking.

4. The subject matter of claim 1 wherein the horn means is a multi-portassembly comprising a plurality of adjacently positioned stationaryhorns for operating in a steady track mode.

5. The structure of claim 2 wherein the switching means comprises amovably mounted flat mirror reflector positioned in intermediaterelation between the scanner means and the horn means for selectablycommunicating microwave energy between the reflector means and eitherthe scanner means or the horn means.

is longitudinally mounted to a shaft to permit selectable rapid rotationof the mirror between two angular positions.

10. The subject matter of claim 9 wherein the shaft is connected to atorquer that selectably drives the shaft between the two angularpositions in response to electrical energization of the torquer.

1. A Schwarzschild antenna system comprising: reflector means forreflecting microwave energy that impinges thereon; scanner meansadjacent the reflector means and communicating with the reflector meansfor producing a wide angle unidirectional sectoral scan; horn meansdisposed in adjacent spaced relationship to the reflector means andcommunicating with the reflector means for producing a narrow anglescan; and microwave switching means mounted adjacent to the reflectormeans to selectively complete microwave communication between thereflector means and either the scanner means or the horn means; wherebythe switching means enables rapid selection of the sectoral scan or thenarrow angle scan.
 2. The structure of claim 1 wherein the scanner meanscomprises a plurality of adjacently spaced feEd horns communicating withrespective waveguide pipes to form an organ pipe scanner which producesa unidirectional scan across the outlet ends of the feedhorns.
 3. Thestructure of claim 1 wherein the horn means comprises at least a singlenutating horn for conical scan tracking.
 4. The subject matter of claim1 wherein the horn means is a multi-port assembly comprising a pluralityof adjacently positioned stationary horns for operating in a steadytrack mode.
 5. The structure of claim 2 wherein the switching meanscomprises a movably mounted flat mirror reflector positioned inintermediate relation between the scanner means and the horn means forselectably communicating microwave energy between the reflector meansand either the scanner means or the horn means.
 6. The subject matter ofclaim 2 wherein each of the feed horns has a pyramidal shape.
 7. Thestructure of claim 3 wherein each of the horn means has a pyramidalshape.
 8. The subject matter of claim 2 and further wherein the hornmeans comprises at least a single nutating horn for conical scantracking.
 9. The subject matter of claim 5 wherein the mirror islongitudinally mounted to a shaft to permit selectable rapid rotation ofthe mirror between two angular positions.
 10. The subject matter ofclaim 9 wherein the shaft is connected to a torquer that selectablydrives the shaft between the two angular positions in response toelectrical energization of the torquer.